Can't speak for the quality of the supplier, but from a bit of Googling (with the secret bonus keyphrase "D8") this place has single blank white eight-sided dice (which I'd guess were 25mm, from US sites offering similar "jumbo" dice) - not sure how easy it would be to colour them yourself, though. Is it important to colour the whole die, rather than to just use different coloured inks to write the numbers or symbols onto them?
This site offers a full set of blank polyhedrals in three colours.